(1) Field of the Invention
The present invention generally relates to evaporative fuel control systems, and more particularly to an evaporative fuel control system for internal combustion engine which stores a fuel vapor from a fuel system in a canister and transfers the stored fuel vapor to an intake system through a purge passage where a purge control valve is provided.
(2) Description of the Related Art
Conventionally, internal combustion engines, especially a gasoline engine for automobile vehicle using a gasoline fuel, have an evaporative fuel control system mounted therein for preventing a fuel vapor supplied from a fuel tank from escaping to atmosphere. This evaporative fuel control system often uses a conventional charcoal canister to store the fuel vapor supplied from the fuel tank, and the fuel vapor stored in the charcoal canister is sent to an intake manifold of the engine via a purge control valve by means of a vacuum pressure generated during engine operation. The engine operation usually when the engine is at low temperatures is not yet stable, and, in such a condition of the engine, a fuel purging by the evaporative fuel control system to make the intake manifold take in the fuel vapor from a purge passage leading to the intake manifold may often worsen driveability. There are several conventional techniques which have been proposed to stop the fuel purging operation of the evaporative fuel control system when the engine is in an unstable state as described above. These techniques are disclosed, for example, in Japanese Published Patent Application No. 57-12021, Japanese Laid-Open Utility Model Application No. 57-55959 and Japanese Laid-Open Patent Application No. 59-192858.
Generally, fuel which is commercially available for use in internal combustion engines, especially in an automobile gasoline engine, may be classified by the distillation characteristic into some categories, for example, a light type fuel, a heavy type fuel and the others. Therefore, in the following description, consider a criterion for classifying these fuels into such categories depending on whether more than 50% of the fuel evaporates at 100 deg. C. The light type fuel meets this criterion and the heavy type fuel does not meet it. The light type fuel generally contains components having low boiling points below 100 deg. C in a greater percent than that of the remainder having high boiling points higher than 100 deg. C. In contrast, the heavy type fuel contains components having high boiling points above 100 deg. C in a greater percent than that of the remainder having low boiling points not higher than 100 deg. C. Especially in a case of an automotive gasoline engine, some different types of fuel having different distillation characteristics may be used for the same engine, sometimes the light type fuel being used and in the other the heavy type fuel being used. When the heavy type fuel is used, the fuel of this type generally is not easy to evaporate when compared with the case of the light type fuel, and there is a greater part of the fuel that is supplied by a fuel injection valve but sticks to an intake manifold wall.
Fuel vapor which is actually introduced into a combustion chamber of the engine is made up primarily of a first part being injected by the fuel injection valve but not sticking to the intake manifold wall, a second part sticking to the intake manifold wall and being in a liquid state, and a third part being on the intake manifold wall in a liquid state but being turned from the liquid state into a vapor state. In the case of the heavy type fuel that has a part sticking to the intake manifold wall in a greater percent, the composition of the fuel having these parts varies greatly for each cycle of engine operation, and a constant amount of each part of the fuel cannot be introduced into the combustion chamber for engine operating cycles, thereby causing a fluctuation of air/fuel ratio within the combustion chamber and an instability of engine operation which are more appreciable than in the case of the light type fuel.
However, a conventional evaporative fuel control system usually stops the fuel purging immediately when it finds that the engine operates unstably, regardless of what kind of fuel is used for the engine. And, immediately when the conventional evaporative fuel system finds the engine in a stable operating state, the system starts the fuel purging at a fixed time with respect to the engine operating cycles, regardless of what distillation characteristic the fuel shows. Therefore, in a case in which the heavy type fuel is used, the fuel purging is started at an excessively early time when the engine operation is not yet stable, causing poor driveability. And, in a case in which the light type fuel is used, the starting of fuel purging delays excessively and too much fuel vapor is adsorbed in the canister, causing poor driveability and increasing undesired fuel odor from a canister opening.